JPS60118359A - Production of weakly deoxidized steel in continuous casting - Google Patents

Abstract

PURPOSE:To obtain a billet for a steel material which has excellent surface quality and is suitable for a mild steel wire rod, etc. by supplying a molten steel contg. a specific amt. of C, silicon, Al and free oxygen into a casting mold lubricated with oil, stiring electromagnetically the molten steel under specific conditions and drawing out continuously the molten steel toward the lower part of the mold. CONSTITUTION:A molten steel contg. <=0.25% C, <=0.1% silicon, <=0.01% Al and the balance iron, unavoidable inpurities and <=85ppm free oxygen is supplied by an open stream or immersion nozzle into a casting mold lubricated with oil. The molten steel is eletromagnetically stirred by an electromagnetic stirrer installed in the casting mold. The conditions for electromagnetic stirring are the AC magnetic field of which the frequency (f) (Hz) and the magnetic flux density G (gauss) at the center of the coil are respectively within the range of the inequality I and the inequality II. The molten steel is then continuously drown to the lower side of the mold and the billet having <=200mm.<2> sectional area is continuously cast.

Description

[Detailed description of the invention] (Industrial application field) The present invention relates to a method for manufacturing weakly deoxidized steel.

(Prior Art) In general, the main characteristics required for steel bars for cold forging and steel materials for wire rods include good surface condition, sufficient deformability, dimensional accuracy, and uniformity. When applying the continuous casting method to the manufacture of such steel materials, if rimmed steel is used, molten steel may overflow from the mold or blow holes elongated in the solidification direction may occur in the solidified shell, resulting in continuous casting. Undesirable for work or as a product. Therefore, it is necessary to deoxidize the molten steel before continuous casting.

Conventionally, methods for deoxidizing molten steel include a vacuum deoxidation method and a method using metal deoxidizers such as 7-erosilicon, ferromanganese, and Ap. If a large amount of metal deoxidizer is used, A
1203-based and SiO2-based inclusions increase, resulting in defects that are undesirable in terms of product quality. For this reason, it is preferable to reduce the content of the metal deoxidizer. Therefore, composite weak deoxidation using Si 2 and Ap as metal deoxidizers or weak deoxidation using vacuum degassing and a small amount of AI are used.

However, in the case of Si-added weakly deoxidized steel using the former deoxidizing method, the problems caused by non-metallic inclusions such as SiO□ and A203 in guild steel with a large amount of deoxidizing agent added are considerably reduced. , the following new problems arise. In other words, in order to add a small amount of Si and to minimize the amount of Ae added, the oxides generated in the steel become so-called silicates whose main components are SiO2 and MnO, and the amount of Ae added is relatively small. tend to remain as large inclusions. and,
It has a negative effect on steel heading.

On the other hand, ultra-low carbon weakly deoxidized steel produced by the latter deoxidizing method has the same problems as quilt steel and Si-added weakly deoxidized steel. Vacuum degassing treatment is characterized by lowering the oxygen and carbon content in molten steel and reducing the amount of gas generated during solidification. The convection of the molten steel weakens, making it easier to solidify while retaining bubbles, causing blowholes and the like, making it impossible to expect a healthy surface condition.

In addition, ultra-low carbon steel has a very high liquidus temperature, resulting in high-temperature tapping in a converter or the like. For this reason, foreign inclusions caused by erosion of bricks in the furnace are added to the inclusions already present in the steel, and the cleanliness of the molten steel is considerably degraded.

JP-A-56-148454 discloses that the continuous casting method is applied to the current problem of low carbon weakly deoxidized steel and is useful for improving quality. Deoxidizing treatment to reduce the amount of free oxygen to the necessary minimum (85 ppm or less)
The low carbon material molten steel is supplied into the mold using an immersion nozzle, and continuous casting is carried out while electromagnetic stirring is carried out in the mold while an insulating mold 7 lux is placed on the surface of the molten metal. thus,
When a rotational force is applied to the molten steel in the mold, most of the free oxygen dissolved in the molten steel changes to gas or oxides, and the rotational force is added to its own buoyancy, causing other floating inclusions to It floats to the surface of the hot water and separates. Electromagnetic stirring is also effective in decarburizing the surface layer of molten steel. The obtained slab has good surface quality, is soft, and has excellent workability and hardenability, and is particularly suitable as a wire bar steel type.

This continuous casting method is used to produce blooms with extraordinary cross-sectional dimensions. However, when manufacturing rods and wire rods, a blooming process must be performed, which inevitably increases costs from the standpoint of energy conservation and yield. Therefore, if the above-mentioned blooming step is omitted by using billet slabs with a small cross section, a significant cost reduction is expected.

A major problem in manufacturing weakly deoxidized steel is the occurrence of blowholes. In other words, since molten steel contains a large amount of free oxygen,
Blowholes are extremely likely to occur on the surface layer or under the skin of slabs, and they remain as flaws even after forging, deteriorating quality. Therefore, it is necessary to almost completely eliminate the occurrence of blowholes.

However, in continuous billet casting, compared to continuous bloom casting, the cross-sectional area of the hot water surface through which the bubbles escape is smaller than the area of the part where bubbles are generated in the unsolidified molten steel, and the bubbles are tends to remain in the slab and form blowholes. For this reason, it is necessary to make the stirring intensity of the in-mold electromagnetic stirring stronger than that of the above-mentioned bloom electromagnetic stirring. By doing so, it is expected that gas generation at the solidification front will be suppressed, and the gas generated will be quickly removed from the high viscosity mushy zone at the solidification front, brought into the low viscosity molten steel, and separated by flotation.

However, when casting molten steel by supplying molten steel through an immersion nozzle and pouring 7 lux onto the surface of the molten metal, as in the continuous casting of Bloom mentioned above, if strong electromagnetic stirring is performed as described above, the stirring strength due to electromagnetic stirring will increase. When the value exceeds that value, the number of inclusions in the slab increases rapidly. This is because the vortex caused by stirring entrains the powder in the mold into the molten steel. Furthermore, in billets with small cross-sectional sizes, slag formation of 7 lacs is not sufficient. In any case, the use of 7 lux is not preferred in continuous casting of billets with relatively small cross-sectional sizes. It goes without saying that when molten steel is supplied into the mold using an open stream, if flux is used, it will get caught up and become inclusions.

(Object of the Invention) An object of the present invention is to provide a method for manufacturing a billet for steel materials suitable for mild steel wire rods etc. with excellent surface quality using electromagnetic stirring.

The present inventors researched a manufacturing method for casting using an oil lubricant (oil casting) and discovered the following method.

(Structure of the Invention) Molten steel containing 0.825% or less carbon, 0.10% or less silicon, 0.010% or less aluminum, the balance iron and other components, and 85 ppm or less free oxygen is produced in an open stream. Or, using an electromagnetic stirring device installed in an immersed environment, the frequency f (Hz) and the magnetic flux density G (Gauss) at the center of the coil are 1.5≦f≦15 and −0,1Of −0,11f 602e The above molten steel is electromagnetically stirred by an alternating magnetic field within the range of ≦G≦2441e, and is continuously drawn out below the mold to be weakly deoxidized.

After the above molten steel is tapped from a converter or electric furnace, it is deoxidized in advance in molten steel processing equipment to reduce free oxygen to 85 ppm.
Control as below.

(Example) After adjusting the deoxidation of molten steel tapped from an electric furnace using a deoxidizing agent such as Al or ferrosilicon using the LF method,
Continuous casting was performed using oil casting using a 125 mm zero billet continuous casting stone. The amounts of the components determined in Tanditsh are in weight %, carbon is O, (16+ Si is 0.06.M
n is 0.32. A/ is 0.003. P is 0.011.
S is 0.012, and the amount of free oxygen is 78 pp
It was m. This molten steel is supplied from the tundish to the ladle using a dipping nozzle while sealing it with nitrogen gas to keep it in an oxidized state. 2.8m/min using oil lubricant
Pull out the slab at a speed of . Electromagnetic stirring was at 5Hz.
It was applied using a rotating magnetic field with a G of 100 ONus.

The electromagnetic stirring conditions are set as follows. In the case shown in Fig. 1 where the frequency f of the alternating magnetic field is 5 Hz, the number of blowholes on the surface of the slab decreases rapidly as the magnitude G of the magnetic flux density at the center of the fill increases and the strength of stirring in the mold increases. ,G
≧350 is 5 or less per 100cn+2.

That is, the lower limit of the appropriate range of G is 350 Gauss.

This decrease is due to the fact that when the oxygen in the molten steel, which became supersaturated during solidification due to insufficient deoxidation, attempts to precipitate as CO gas, the flow of the molten steel due to stirring prevents it from being trapped in the solidified layer. be.

8- Furthermore, as the stirring intensity increases, the amount of equiaxed crystal nuclei generated by cutting columnar crystals increases, and the score of the central cavity in the slab is also improved.

On the other hand, since the concentrated molten steel in the Matsushi zone is washed away by the molten steel flow, the solidified layer part that has been stirred becomes negatively segregated, and the degree of negative segregation (=(CwB-co)/Co, where Co
: C concentration in molten steel "WB" (minimum C concentration in the negative segregation zone generated by being stirred) increases with increasing stirring power. If the negative segregation degree of the surface layer increases too much, there will be a problem of insufficient hardness of the surface layer in products that undergo a heat treatment process.
It is necessary to suppress the negative segregation degree to -0.2 or less. Therefore, the upper limit of the appropriate range of G is 1400 Gauss.

Therefore, when f=5Hz, the appropriate range for stirring in the mold is 350≦G≦1400.

If this appropriate range is set within the range of 1.5≦f≦15, the two curves shown in Fig. 2 are obtained, and as the frequency f increases, both the upper and lower limits of the appropriate range decrease, and the range increases. I found it to be narrower. The upper curve is 2441e-0°11
approximation 1 at f, and the lower curve is approximately 602e '°
The approximation ends at 10f.

Therefore, the area where 1.5≦f≦15 and 602e-0°10f≦G≦2441e-Oollf was set as the appropriate range.

In the billet cast under the above conditions, the number of blowholes is 5 or less per 100 cm2, and these blowholes are so small that they cannot be detected unless the slab is pickled to completely remove scale. That is, the surface quality of the billet was good. This surface quality is equivalent to that of a weakly deoxidized steel bloom continuously cast material manufactured by the manufacturing method described in the above-mentioned prior art.

In the case of oil casting, since reoxidation of the molten steel is large, it is desirable to use an Ar seal or the like to perform oxidation-free casting between the ladle, tundish, and mold. In this example, sealing was performed using nitrogen gas.

Further, in this example, molten steel tapped from an electric furnace was treated by the LF method. Generally, after steel is tapped from a converter or electric furnace, free oxygen may be controlled to 85 ppm or less using molten steel processing equipment that uses Ar stirring, vacuum degassing, or the like.

In addition, in the above example, the molten steel contained 0.06% of C, but if it was 0.25% or less, the same quality as a large cross-section bloom slab could be produced under the same application conditions. Obtainable. Although the illustration is omitted, this is obvious in view of the well-known dependence of the properties of molten steel on the C concentration.

(Effects of the Invention) According to the present invention, billets of low carbon weakly deoxidized steel with excellent surface quality can be manufactured.

[Brief explanation of drawings]

FIG. 1 is a graph showing the appropriate range of electromagnetic stirring. FIG. 2 is a graph showing the frequency dependence of the appropriate range. Patent Applicant Kobe Steel Co., Ltd. Agent Patent Attorney Kotobuki and 2 other people V1 (I 小騨鼾Y侭に嘗H屹t,uu'Oo)／）I 柟WY1'-平-' 1! −”
J'! + (JJlr Strive ← Y謔tomlP～SuCho,
￥

Claims (1)

[Claims] (1) In continuous casting of slabs having a cross-sectional area of 200 mm or less, carbon of 0.25% or less, silicon of 0.10% or less, aluminum of 0,00% or less, and the balance iron and other components. and 85 ppm or less of free oxygen using an open stream or submerged nozzle,
Using an electromagnetic stirring device that is supplied into an oil-lubricated mold and installed inside the mold, the frequency f (Hz) and the magnetic flux density G (Gauss) at the center of the coil are 1.5≦f≦15, respectively. -o, i or -o, i 1r 602e ≦G≦2 44 1e In continuous casting, the above-mentioned molten steel is electromagnetically stirred by an alternating magnetic field within the range of A method for manufacturing weakly deoxidized steel. (2. In the method for producing weakly deoxidized steel in continuous casting described in claim 1, the above-mentioned molten steel supplied to the mold is preliminarily stirred with argon or heated after being tapped from the converter or electric furnace. A method for producing weakly deoxidized steel in continuous casting, characterized in that the molten steel is deoxidized in a molten steel processing equipment using vacuum degassing and free oxygen is controlled to 85 ppm or less.